The present invention concerns a device for the oriented illumination of a flat surface by means of a solid optical guide, provided with microprisms a particular arrangement of which corrects the light rays originating from at least one source allowing said rays to be injected at least at one of its ends, so as to obtain along the entire length of the guide a uniform grazing emergence via a face directed towards said surface to be illuminated.
The invention concerns more particularly a device of this type for illuminating surfaces of small dimensions, such as display devices for measuring apparatus and timepiece dials.
Devices in which a surface is illuminated by an optical guide made of a flexible element, of a translucent material usually having a generally rectangular cross-section are already known. Such a guide allows the light injected at one end to be led along the guide by total internal reflection (TIR) and allows the rays which, inside the guide, strike the surface directed towards the surface to be illuminated, to be extracted to said surface to be illuminated at an angle less than the critical angle xcex8c, whose value depends upon the refractive index n of the material used. In order to increase. the number of rays corresponding to the aforecited criteria, it is also proposed to coat the other faces which are not directed towards the surface to be illuminated with a reflective white varnish. Such devices correspond for example to that which is disclosed in European Patent No. EP 0 860 755 for the artificial lighting of a timepiece dial by means of a diode. These devices have the drawback of having poor yield because of losses by absorption on the other walls, and having a luminous intensity which decreases progressively as one moves away from the light source.
In order to reduce the loss of light energy, various devices propose using microprisms which work either in reflection, or in transmission. When the microprisms are used in reflection, they are arranged on a surface of the guide which is perpendicular to its longitudinal line so as to return the incident rays by total internal reflection onto the opposite face through which the rays can emerge to illuminate a surface. When the microprisms are used in transmission, they are arranged perpendicular to the longitudinal line of the guide on the face though which the rays have to emerge to illuminate a surface. This latter configuration corresponds for example to the devices described in U.S. Pat. No. 5,555,109 and European Patent No. EP 0 666 247. Whether the microprisms are used in reflection or in transmission, it will be observed that they are always arranged on faces of the guide which are substantially perpendicular to the surface to be illuminated. In the case of a curved guide, and in particular in the case of a guide in the shape of a ring, the structuring of the microprisms is made more difficult technically, which naturally leads to an increase in the cost of the illumination device. It will also be observed that these devices of the prior art still do not provide a satisfactory solution to the orientation of the emerging rays towards the surface to be illuminated. Indeed, many rays go directly from the guide to the observer and dazzle him instead of illuminating the surface.
An object of the present invention is to overcome the drawbacks of the aforementioned prior art by providing a device for illuminating a flat surface by a microprism guide, said microprisms being structured in a plane face of the guide which is substantially parallel to said surface in accordance with an arrangement allowing the emerging rays along the guide to be oriented in uniform grazing incidence towards said surface, which will thus be easier to observe as a result of a reduction in the dazzle effect.
The invention therefore concerns an oriented illumination device for a planar surface by at least one lambertian source by means of a solid optical guide formed of a succession of elementary blocks propagating the light along an axis Oz and including at least one microprism, said guide being made of a material with a high refractive index n. This refractive index defines inside the guide a light propagation cone whose angle of revolution corresponds to critical angle xcex8c. The cross section of the guide can have any contour, but has at least one plane angle xcex4 one side of which generates a first plane face of the guide substantially parallel to the surface to be illuminated and the other side of which generates a second plane face substantially perpendicular to the surface to be illuminated. The other surfaces generated by the other parts of the cross section can also contribute to improving the extraction of the injected light, for example in accordance with the techniques explained in the preamble, but their configuration does not directly concern the invention.
The invention is characterised in that the microprisms are structured in a hollow on the first face of the guide substantially parallel to the surface to be illuminated and in that each microprism has one of its plane facets oriented towards the light source by being inclined towards the second plane face of the guide so as to orient the middle ray emerging from said second face at an angle xcex81, with respect and towards said surface to be illuminated. The inclination of this facet of the microprism concerned is defined on the one hand by an angle {right arrow over (xcex1)} determined in the plane of the first face of the guide by an edge of said facet and by the axis of propagation Oz of the light in an elementary block, and on the other hand, by an angle {right arrow over (xcex3)} determined in the plane of the second face of the guide by the edge of said facet and by the direction perpendicular to the surface to be illuminated. Angles {right arrow over (xcex1)} and {right arrow over (xcex3)} are determined, on the one hand from the refractive index n of the material used for the guide which will determine inside the guide the angle xcex8c/2 of the incident middle ray on the microprism facet and on the other hand, from the average angle of incidence xcex81, selected so that the emerging rays illuminate the whole of the surface in question. This average angle of incidence xcex81 is obviously a function of the selection made for the respective dimensions of the guide and said surface to be illuminated, and finally of the value of the plane angle xcex4 of the cross-section of the guide.
The values of angles {right arrow over (xcex1)} and {right arrow over (xcex3)} can also be calculated, as a function of the aforecited physical characteristics of the device, from derived parameters, such as the direction cosines of the reflected or refracted light rays, said values being carried over in a orthogonal universal set x, y, z parallel to the cross-section of each microprism and to the surface to be illuminated.
It is possible to give plane angle xcex4 of the guide cross-section at each microprism any value one wishes, the preferred value being Π/2, this value also being retained for a better understanding of the following detailed description.
Angle xcex4 may vary with respect to this reference value (Π/2) by approximatelyxe2x88x9210xc2x0 (trigonometric direction), either by keeping the first surface strictly parallel to the surface to be illuminated, or by keeping the second surface strictly perpendicular to the surface to be illuminated. In other words, angle xcex4 may vary substantially between 80xc2x0 and 1000.
As indicated previously, the guide may be considered, for a better understanding of the invention, as a succession of rectilinear elementary blocks B, able to include one or more microprisms as defined above, and the linking of which can either form any longitudinal line, or a rectilinear line, or a regular curved line, such as a circle. In the case of a curved line, angle {right arrow over (xcex1)} is defined with respect to the tangent of the point of the guide being considered.
In order to obtain uniform illumination of the surface along the guide, in which there exists a reduction in intensity of the light flux because of the ejected light, but also because of losses due to reflections, absorption by the  less than  less than non active  greater than  greater than  faces, as well as absorption by the material used to form the guide itself, it is desirable, according to a preferred embodiment of the invention, to compensate for this reduction in light flux by increasing the density of the microprisms as one moves away from the light source.
According to a first preferred embodiment, the microprisms are structured over the entire width of the guide and the number thereof by elementary block Bi increases as one moves away from the light source. The closest elementary block to the light source will for example include one microprism, and the one furthest away will include four.
According to a second preferred embodiment, each elementary block Bi includes only one microprism, but the length of the microprisms increases the further they are from the light source.
These two embodiments are of course compatible with each other, and it is possible to implement them both simultaneously.
The microprisms are formed on the first face of the guide by direct or indirect engraving, such as mechanical machining with suitable diamond tipped tools, etching through photoresist masks, by chemical means or by laser beams, said techniques being cited by way of non-limiting examples.
It is also possible to produce said microprisms by replicating from an engraved die which is fitted to an injection moulding machine to obtain the guides by moulding, or which is used for stamping a surface in which one wishes to make said microprisms. Whichever technique is used, the microprisms can be made either directly on the guide, or on a refractive index plate substantially identical to that of the guide and having the same contour as the first face of the guide, said plate being then applied onto said first face, for example by means of an adhesive material.
The microprisms thus formed can be left as they are, i.e. not.undergo any additional treatment, and change the direction of the light vector simply by total internal reflection. As a function of their angle of incidence, the rays which would exit the guide by refraction on said facet can be reoriented inside the guide as a function of the inclination which has been given to the second facet of each microprism.
According to an alternative embodiment which allows the light flux used for illuminating the surface to be increased, it is also possible to coat the facet of each microprism oriented towards the light source with a reflective coating. This advantage is however offset by the resulting increased manufacturing cost (selective coating of a very small surface) and by the losses in light flux due to absorption by the reflective surfaces. If the surface where the microprisms are structured is completely coated, the increase in manufacturing costs is slightly reduced, but the losses by absorption of the reflective surfaces is also increased. The choice between these different embodiments thus essentially depends on the respective dimensions of the guide, the microprisms and the surface to be illuminated.
As regards the positioning of the light source, it can be selected from those known in the prior art, with a source along the axis of the guide, or better, with two sources arranged at each end, the guide then having a symmetrical microprism structuration with respect to its median point. It is also possible and economically advantageous to arrange a single source perpendicular to the surface to be illuminated facing a dihedron formed in the first face of the guide substantially parallel to the surface to be illuminated, said dihedron having an angle such that it allows the light flux to be injected by total internal reflection into two half-sectors of the guide. The emission cone of the source being formed of light rays having different angles of incidence and able to be refracted, and thus lost, according to another aspect of the invention the aforementioned dihedron is replaced by a double dihedron, or a polydihedron forming a curved surface by a succession of infinitesimal elements.
The microprism illumination device which has just been described allows any type of surface to be illuminated. However, its preferred application is the illumination of surfaces of small dimensions such as the display devices of measuring apparatus and timepiece dials.